The present invention is generally directed to an apparatus and method for in situ, real time measurements of properties of a liquid such as, for example, a molten metal. The liquid may be stationary or in a flowing state. Real time measurements may be taken from any location including inside the liquid and on the surface of the liquid. When measurements are taken below the surface of the liquid, a stable volume of an inert gas under continuous flow may be provided at the interface of the apparatus and the liquid to enable a rapid and accurate passage of a radiation beam into the liquid to generate a detectable species which is then analyzed to determine the desirable properties. Alternatively, the apparatus may operate in a passive mode without any supplied radiation by detecting species emanating from the liquid.
The measurement of various properties of a liquid including, but not limited to, quantitative and qualitative measurements such as concentration and composition is of critical importance in a variety of industrial applications. When a liquid is contained within a vessel, measurements can be routinely taken by obtaining a sample of the liquid and transporting the sample to a remote location such as a laboratory so that the sample may be analyzed. Quantitative and qualitative measurements can be taken at the laboratory and then transmitted back to the operator of the vessel to determine if adjustments to the composition of the liquid must be made. While instrumentation is well known in the art to measure concentration and composition of a liquid, the time it takes to make such measurements and to relay the information to the operator of the vessel can be critical to productivity as in the metal (e.g. the production of steel or aluminum) and glass industries.
As an example, closely controlling the composition of steel during its manufacture is critical to the production of quality products. It is incumbent upon the operators of the steel plant to fine tune the composition of the molten steel. Currently, samples of molten steel are taken from the furnace, transported to a laboratory where spectrometric measurements are taken that determine the elemental composition of the steel. The results of the analysis are transmitted back to the furnace operator who determines whether the actual composition of the molten steel is the same as that desired. If not, adjustments to the composition may be made by adjusting the relative amounts of the components of the molten metal.
The time it takes to complete the compositional analysis of the molten product therefore is critical to the rate of production of the desired product (e.g. steel). It therefore is desirable to employ an apparatus and method for in situ analysis of liquids such as molten metals and glasses so that adjustments to the composition of the liquid may be made in a shorter period of time than through the use of outside labs. One such approach is disclosed in Carlhoff et al. (U.S. Pat. No. 4,995,723 and related U.S. Pat. No. 4,993,834) incorporated herein by reference. These references disclose a method of analyzing elements of a molten metal by providing a stationary conduit at a side wall of the vessel containing the molten metal. A laser beam is directed into the conduit and onto the surface of the molten metal. The light generated by the plasma formed by the interaction of the laser beam and the molten metal is coupled with an optical waveguide through a lens system and then introduced by the optical waveguide into a spectrometer. The system provides for measurements of the molten metal on the surface only and does so only at a fixed point due to the stationary position of the conduit.
Another stationary conduit system is disclosed in Cates (U.S. Pat. Nos. 5,830,407 and 6,071,466) incorporated herein by reference. A stationary conduit is inserted into the bottom of a vessel containing a molten metal. The center pipe of the stationary conduit carries a transparent gas under pressure to maintain an opening in the molten metal. The gas flow has a sufficiently high hydrostatic head to prevent the molten metal from entering the conduit. A sight glass assembly enables a direct view of the molten metal and an optical sensing device such as a photometer or spectrometer is employed for determining the composition of the molten metal. Here again, measurements of the molten metal are taken from a fixed position at only one location within the molten metal.
The systems described in the above-mentioned references suffer from a number of disadvantages. These prior art systems employ stationary conduits which require all measurements to be made from a fixed location either only on the surface of the molten metal or only at one location within the molten metal. Such systems are disadvantageous because the molten metal may vary in composition within a single vessel. The accuracy employed in adjusting the composition of the molten metal depends in part on getting a highly accurate reading of the entire composition of the molten bath. If only one fixed location for analysis is provided as in the above-mentioned references, the accuracy of the analysis with respect to the entire molten metal is compromised.
Further disadvantages of the above-mentioned prior art relate to the angle at which the instruments interact with the probe. Because the molten material is of higher density than the gas, the device disclosed in Carlhoff et al., cannot sustain a static bubble of gas for making measurements. The heavier molten material will flow into the hole in the furnace wall displacing any gas. Therefore, gas must be flowing continuously in order to keep the molten material out of the instrument. This continuous flow will result in a non-stationary interface between the gas and the molten material, greatly complicating the measurement process, which is most accurate when the interface is stationary so that the optics are in focus. Since constant pressure is required to keep the molten material out of the device, the loss of gas pressure due to for example, a leak in the gas supply line, may adversely affect the desired measurements and may result in damage to the instruments.
The device disclosed in the Cates references has similar disadvantages. While a vertical orientation can maintain a static surface, if pressure is lost, the device will be destroyed and the molten material lost, just as in the case of Carlhoff et al. Also, while the vertical column of gas can be stationary in Cates, it is unstable, particularly if some of the gas is released into the bath due to a disturbance of sufficient magnitude, the remainder of the gas is likely to follow, and molten material will flow into the tube.
A further disadvantage of Cates concerns the required access from the bottom of the furnace. It is typically very difficult to gain access to the bottom of commercial furnaces because of the weight of the furnace. Also, there is the potential for a disastrous leak of molten material onto the factory floor with a port located on the bottom of the furnace. When the port is on the side, material will leak out only until the level of molten material in the vessel falls below the level of the port. With the port positioned on the bottom of the vessel, and the column containing the gas extending only a short distance into the furnace, nearly the entire volume of molten metal contained in the furnace can leak out if there is a loss of gas pressure.
Another disadvantage in the Cates and Carlhoff et al., systems relates to the location where analyses are performed. Carlhoff et al., samples the molten material at the wall of the furnace, and Cates samples the molten material near the bottom of the furnace. These locations may contain molten material that is not representative of the bath as a whole. When the furnace operators introduce alloying elements into the bath, they attempt to mix them thoroughly throughout the bath. However, it is most difficult to ensure thorough mixing of the ingredients close to the side walls of the furnace where it is difficult to introduce mechanical agitation. If the alloying elements are diffusing throughout the bath, it wall take the longest period of time for them to reach the walls. If the melt is poured before diffusion is complete, sampling near the walls will not be representative of the bath as a whole. Also, in the case of molten glass there are large thermally generated currents, such as rising pockets of hotter material and descending flows of cooler material. These rising and falling currents tend to prevent or render the production of the homogenous melt more difficult.
Other approaches to making quantitative and/or qualitative measurements of a metal employ laser induced breakdown spectroscopy (LIBS) systems. Such systems generally provide an apparatus for in situ real-time spectroscopic analysis of a material through the employment of laser pulses of sufficient power to irradiate a representative quantity of a heterogeneous sample to the extent that it forms a plasma. The plasma is composed of a small amount of the material which has been vaporized and ionized by the laser pulses. In the plasma, the molecules of the material are dissociated and the atoms are excited into charged states. As the plasma cools, the charged atoms (ions) emit electromagnetic waves in wavelengths specific to the atom of the particular element. By observing the electromagnetic radiation with a spectrometer capable of resolving the different wavelengths, the elements in the radiated sample can be identified. Quantifying the intensity of the radiation and comparing it to reference samples, and/or through calculations using various atomic constants, the concentration of the atomic elements in the material can be ascertained.
Examples of such systems used to analyze solid samples are disclosed in Sabsabi et al., (U.S. Pat. Nos. 5,781,289 and 6,008,896). Eivindson (U.S. Pat. No. 5,664,401) analyzes molten metal through this method by applying a LIBS system to the gas above the surface of the molten material and inferring the composition of the melt from the measured composition of the gas. Singh et al., (U.S. Pat. No. 5,751,416) describes a method of analyzing a liquid material directly using a LIBS system but only on the surface of the liquid. Each of the above-mentioned references is incorporated herein by reference.
Kim (U.S. Pat. No. 4,986,658), incorporated herein by reference, describes a method of analyzing molten metals below the surface of the melt using a LIBS system. The reference design suffers from a number of drawbacks. Firstly, the expensive components of the device (i.e. the spectrometer and the laser) are placed very close to the furnace. In the event of an accident in which these components either fall into the furnace or otherwise come into contact with the molten metal or are exposed to temperatures above the tolerance threshold of the components (e.g. a failure in the cooling system), the investment in the instrument is lost. This danger complicates the reference design, requiring the use of heat shields and coolant systems. This complexity adds to the cost of the system and increases the number of subsystems that can fail, leading potentially to a loss of the equipment. Also, the probe is limited to a few centimeters below the melt surface so as to void any floating slag. The system is also incapable of being immersed to any appreciable extent within the furnace.
Another consequence of the reference design is the limited number of locations the melt can be sampled. Because of the placement of the spectrometer and laser just above the molten material, the device of the reference ""658 patent can not probe into deeper locations of the melt without substantially changing the design, such as lengthening the exterior of the probe and changing the focal lengths of the optical components. Also, the probe cannot be inserted at an angle other than vertical, further limiting the applicability of the design only to furnaces that have access from the top of the furnace. As detailed above, sampling the molten material at multiple locations may result in substantially better analysis of the melt.
Another drawback of the ""658 patent device is that it prevents deployment of multiple probes throughout a metal manufacturing facility. By coupling the laser and spectrometer to the probe housing, deploying multiple probes would force the plant operator to purchase many lasers and spectrometers, a prohibitive expense.
The reference device is further disadvantageous because it does not flow inert gas into the molten material. There are two benefits to flowing inert gas into the melt. First, the inert gas can locally stir the melt so that each laser pulse does not result in an analysis of the same material. Second, the flow of inert gas can remove contaminants from inside the molten material. This occurs in two ways. Undesirable gases dissolved in the melt are mixed into the inert gas as it rises and are thereby removed from the melt. Also, contaminants that would ordinarily float to the top of the melt to be skimmed off are agitated by the bubbles and rise more quickly than they would otherwise. The practice of using an inert gas to remove contaminants is well known to those versed in the art of molten metal production.
It would therefore be a significant advance in the art of performing in situ real time measurements of a liquid if an apparatus could be provided which is capable of measuring at least one property of a liquid regardless of whether the liquid is stationary or flowing, and is mobile, so that the apparatus can rapidly sample the liquid at multiple locations within the liquid or on the surface thereof. It would be a further advance in the art to provide an apparatus which can be inserted into the liquid at different angles with respect to the surface of the liquid and which neither the liquid or the components of the apparatus will be lost if there is a system failure.
It would be a still further advance in the art to provide an apparatus and method for making such measurements in which there is greater efficiency in the use of relatively expensive components (e.g. laser) and the elimination of unnecessary duplication of such components.
It would be an additional advance in the art to provide an apparatus and method for making such measurements in which if the apparatus is submerged in the liquid, the supply of inert gas to the liquid may assist in a) purifying the liquid, b) stirring the liquid so that a more representative sample of the liquid is analyzed, and c) preventing interference with the measurements by not generating bubbles of gas within the liquid that interfere with the sensor""s operation.
The present invention is generally directed to an apparatus and method for in situ, real time qualitative and/or quantitative measurements of properties of a liquid such as, for example, a molten metal. The liquid may be stationary or in a flowing state. The invention is capable of taking such measurements from the surface of the liquid and from any location inside the liquid. When used below the surface of the liquid, a stable volume of an inert gas under continuous flow may be provided at the interface of the apparatus and the liquid to enable a rapid and accurate passage of a radiation beam into the liquid to generate a detectable species, which is then analyzed to determine the desired properties. The apparatus may, instead of a stable volume of inert gas, employ a window as a barrier to the liquid entering the apparatus. Alternatively, the apparatus may be passive, detecting species emanating from the liquid without any radiation supplied by the apparatus.
In one aspect of the present invention, there is provided:
An apparatus for measuring at least one property of a liquid at or below the surface of the liquid comprising:
a) a housing having a forward end;
b) at least one probe assembly at the forward end of the housing, said probe assembly comprising an inert gas generating means comprising a source of inert gas, a conduit for channeling the inert gas to the forward end of the housing and means for providing, under flow from the source of inert gas, a stable volume of inert gas at the interface of the forward end of the housing and the liquid;
c) a radiation beam assembly comprising means for generating a beam of radiation sufficient to vaporize a portion of the liquid into a detectable species, means for transmitting the radiation beam through the forward end of the housing to the interface of the liquid and the stable volume of inert gas; and
d) detection means for receiving the detectable species and for detecting from said detectable species at least one property of the liquid.
Methods of measuring properties of liquids at or below the surface thereof using the apparatus are also encompassed by the present invention.
In another aspect of the invention, a window is provided at the end of the probe assembly through which the laser beam is transmitted thus eliminating the need of providing at stable volume of gas at the interface with the liquid to be analyzed.